The two current ELISA standards ER2 and ER3 are labelled at 12 and 6 units, respectively. Extensive guinea pig potency testing on these standards is being performed to calculate the exact unitage to the 0.5 unit level. ER2 is being used as the standard in the Tetanus ELISA assay to determine the titer of routine lot release serum. Sera from DTPads lots stored frozen in the past will represent one set of samples. These sera were saved, because they were borderline samples in the official potency assay. In addition, samples will be collected to calculate both the individual and pool titers of each new DTPads immunized guinea pig. The pool will be tested in the official potency assay. All Tetanus ELISA assays are being performed using an automated dilutor. A U.S. Master Standard for the Tetanus ELISA is being considered. Guinea pigs will be immunized with U.S. Standard Tetanus Toxoid Lot 1 and serum collected. The individual immune response over a 4, 6 and 8 week period is being examined using the ELISA assay. This information will give us insight into the number of guinea pigs to be used and the optimal bleed time when formulating the U.S. Master Standard. ER3 will be used for lyophilization and stability studies. Some of the variables to be considered are: 1) The starting concentration to be used in the Tetanus ELISA assay. 2) The stability of the antitoxin after being lyophilized. 3) The amount of material used to quality control the antitoxin before and after being lyophilized. 4) Pre-processing of the antitoxin before being lyophilized. 5) The type of vial and volume used. 6) The number of vials shipped per year.